Bluetooth: Skip vendor diagnostic configuration for HCI User Channel

When the HCI User Channel access is requested, then do not try to
undermine it with vendor diagnostic configuration. The exclusive user
is required to configure its own vendor diagnostic in that case and
can not rely on the host stack support.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>